This invention relates to a startup procedure for polymerization systems using multiple catalyst compounds that reduces the amount of undesired product produced during start-up.
Higher molecular weight generally confers desirable mechanical properties and stable bubble formation onto olefin polymers. However, it can also inhibit extrusion processing by increasing backpressure in extruders, promoting melt fracture defects in the inflating bubble and potentially, promoting too high a degree of orientation in the finished film. To remedy this, one may combine a second lower molecular weight polymer with the first to reduce extruder backpressure and inhibit melt fracture. This combination can be achieved by blending the polymers physically or by co-producing them at the same time. For example, Mobil, in their patent application WO99/03899, discloses using a metallocene type catalyst and a Ziegler-Natta type catalyst in the same reactor to produce a processable high density polyethylene.
Other dual catalyst systems have been used in the past for a variety of reasons. For example WO 98/02247 discloses a dual catalyst system of a metallocene and a non-metallocene (TiCl4+alcohol) treated with the contact product of dialkylmagnesium and trialkylsilanol. WO 98/02247 discloses dual metallocene systems and describes the idea that the two different transition metal sources exhibit a different hydrogen response under the same polymerization and hydrogen conditions as critical. (Hydrogen response is the sensitivity of the catalyst to manipulation by adding or subtracting hydrogen to or from the polymerization system to produce different products.) Likewise, U.S. Pat. No. 4,935,474 discloses olefin polymerization in the presence of two or more metallocenes (activated with alumoxane) each having a different propagation and termination rate constants. Liquid mixtures of many classes of catalysts are disclosed for use in gas phase polymerization in U.S. Pat. No. 5,693,727. U.S. Pat. No. ""727 discloses that more than one liquid metallocene may be employed. Similarly, EP 0 770 629 A discloses a process to produce bimodal polymers using two reactors in series. In some circumstances only the reaction conditions and monomer feeds are changed in the second reactor. In other circumstances a second different catalyst is added to the second reactor.
Hence, one-step polymerization processes to produce bi-modal polymers have become very desirable because of their perceived efficiencies in production and cost. These methods are more difficult to start up however, because two different catalysts, sometimes having very different reactivities and kinetic profiles, need to be stabilized during start-up. One method to do so is to bring the first catalyst on-line, stabilize it, then introduce the second catalyst and allow the system to stabilize. This has the disadvantage of requiring significant amounts of time, however, and producing significant amounts of undesired polymer. Nor can this method be used for dual catalyst systems in which both catalysts are co-deposited on the same support material, or intimately co-mixed in solution or spray-dried formulations. In some instances prior to start up, the polymerization reactor is charged with an initial polymer bed comprised of the product to be produced. Surprisingly, this approach by itself is not sufficient to eliminate undesired product produced during start up.
WO 99/31142 discloses a start up method for a gas polymerization reactor using a Ziegler-Natta type catalyst that increases the partial pressure of the olefin and the catalyst introduction rate into the reactor whilst maintaining the ratios of the partial pressures of the olefin to the hydrogen and to any co-monomer present. The method disclosed by WO 99/31142, however, cannot be applied to one-step polymerization processes using dual catalyst systems because it fails to consider the effect of the prescribed rate of increase in the olefin partial pressure and the catalyst introduction rate on the product properties of the polymers being manufactured. More specifically, in order to minimize the amount of undesired product produced during the startup of a one-step polymerization process using dual catalyst systems, it is beneficial to consider the reactivities or kinetic profiles (or both) of each catalyst comprising the dual catalyst system in concert with other characteristics of the polymerization process, such as initial polymer bed composition, target production rate, and residence time.
An important product property that may vary during the startup of a one-step polymerization process using dual catalyst systems is the split or relative mass of polymer in the product that has been produced by each of the catalyst components. Because this is often a product characteristic with narrow specification limits, especially for products with bimodal molecular weight distributions, it is desirable to control the relative amounts of polymer produced during all phases of the manufacturing process, including startup and transitions.
The present invention describes a process for reducing the amount of undesired product produced (for example a product whose split of the relative amounts of the polymers is outside the specification limits) during start up of continuous one-step olefin polymerization processes carried out with dual catalyst systems in well-mixed reactors.
The instant invention preferably manipulates the rate of introduction of catalyst into the polymerization reactor during start up using pre-defined trajectories, and prescribes methods for defining and evaluating these trajectories in a manner that accounts for the kinetic profile of the catalyst system in concert with other characteristics of the polymerization process.
This invention relates to a method to start up an olefin polymerization process comprising:
a. calculating a trajectory, from elements including catalyst deactivation rate constants (kd), for the rate of introduction of a catalyst system, into a reactor, said catalyst system comprising two different metal catalyst compounds (A and B) and at least one activator, wherein the ratio of the deactivation constants of the two different metal catalyst compounds kdA/kdB is not 1; and
b. introducing olefin monomer, a catalyst system, optional co-monomer, and optional chain transfer or termination agents into a polymerization reactor in a manner such that the catalyst system introduction rate is manipulated to follow the trajectory until a desired production rate is achieved.
The term xe2x80x9cstart upxe2x80x9d is used herein as it is used in the art (for example, as used in WO 99/31142) to refer to that time period when a desired product is first targeted, wherein polymerization system changes commence or are first made to achieve the desired product, up to that time when no additional substantial changes are intended and the polymerization system stabilizes on producing the desired product at maximized production rates.
The rate of introduction of the catalyst system is preferably controlled so as to follow a pre-determined trajectory that has been selected as described below. In a preferred embodiment other reactor conditions such as pressure and temperature remain substantially constant. By xe2x80x9csubstantially constantxe2x80x9d is meant that the reactor condition in question does not vary enough to alter the product produced, typically the variation is less than 10%, preferably less than 5%, preferably less than 3%, more preferably less than 1%.